Fix for compilation error about missing virtual destructor in GCDQueue (#236)

Summary:
error: destructor called on non-final 'facebook::sonar::GCDQueue' that has virtual functions but non-virtual destructor [-Werror,-Wdelete-non-virtual-dtor]
    __data_.second().~_Tp();
Pull Request resolved: https://github.com/facebook/flipper/pull/236

Reviewed By: passy

Differential Revision: D9351946

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 4718460ff587e1d5fa475a5f3bf115b1e2b5305f

iOS/Plugins/SonarKitNetworkPlugin/SonarKitNetworkPlugin/SKDispatchQueue.h

@@ -30,6 +30,8 @@ namespace facebook {
         dispatch_async(_underlyingQueue, block);
       }
 
+      virtual ~GCDQueue() { }
+
     private:
       dispatch_queue_t _underlyingQueue;
     };